docs: def _get_default(cls, attribute):
          """Return the default value for `attribute`.
  
          If no default is specified in this style, get the default from the
          nearest superclass.
          If `attribute` is not supported, raise a :class:`KeyError`."""
          try:
              for klass in cls.__mro__:
                  if attribute in klass._attributes:
                      return klass._attributes[attribute].default_value
          except AttributeError:
              raise KeyError("No attribute '{}' in {}".format(attribute, cls))